The invention relates to a semiconductor device with a semiconductor body comprising a layer-shaped region of a first conductivity type which adjoins a surface and which merges into a subjacent semiconductor region, called substrate hereinafter, of the opposite, second conductivity type via a pn junction, which semiconductor body is provided with a high-voltage transistor of the lateral DMOS type, comprising a source zone of the first conductivity type adjoining the surface, a base region of the second conductivity type surrounding the source zone in the semiconductor body and also adjoining the surface, and a drain zone of the first conductivity type which adjoins the surface and which is situated at a distance from the base region and is separated therefrom by an interposed portion of the layer-shaped region, the source and drain zones of the transistor forming an interdigitated configuration with at least three elongate zones of the first conductivity type lying next to one another, of which the central zone forms a drain zone with at least an end face and of which the two outermost zones situated on either side of the central zone each form a source zone.
Such a device is known from inter alia U.S. Pat. No. 5,523,599. An n-channel DMOST is described therein formed in an n-well at the surface of a p-type substrate. The transistor has an interdigitated source/drain configuration so as to obtain a high current-carrying capacity. As is shown inter alia in FIG. 7 of U.S. Pat. No. 5,523,599, the high-ohmic n-well, which at the same time forms the drift region of the transistor, is interrupted at the area of the tips of the drain fingers and forms a pn junction with the p-type substrate, which is also high-ohmic, at the area of the tips of the drain fingers. A higher breakdown voltage is obtained thereby, given a high voltage applied to the drain, than if the n-well were not interrupted. In the latter case, indeed, the n-well would form a pn junction with the p-type base region at the area of the fingertips with a lower breakdown voltage owing to the higher doping level of the base region. A disadvantage of this embodiment is that the p-type base region is connected to the p-type substrate in an electrically conductive manner, whereas it is desirable for a number of applications that different voltages should be applied to the substrate and the base region. In addition, the n-type surface region is not formed by an n-well which is locally provided in many processes, but by an epitaxial layer which extends over the entire surface area, which renders it difficult to form a drift region with the configuration described above.